1. Field of the Invention
This invention relates to apparatus and methods for driving a thermal printing head and, more particularly, to elements of the printing head.
2. Description of the Prior Art
The printing head apparatus commonly used in thermal printing consists of an array of resistive heating elements selectively active under the control of digital circuitry which may be mounted on the printing head substrate. Heat from each selected element produces a printed "dot", and all of the selected elements cooperate to produce a printed line pattern directly on heat sensitized media, or via a heat sensitive ribbon in the case of thermal transfer printing. As the printer mechanism moves the medium perpendicular to the printing head, the array of heating elements is repeatedly loaded with data and activated to print a sequence of lines to produce a printed image.
The image information consists of a binary data stream which is loaded into a data shift register in serial fashion. Once loaded, each data bit controls a single heating element. A strobe signal activates all the heating elements that have a corresponding control bit that has been set to a logical value of 1. The energy received by the activated heating elements is controlled by the length of the strobe signal and the voltage applied to the elements, which voltage is the same for each element. It is often necessary to have some heating elements receive more energy than others. Thus, if a particular heating element has been recently heated, it will retain some of that heat and require less energy to produce a well-printed dot. Alternatively, a heating element that has been not been heated recently will require more energy to produce the same well-printed dot. As print speeds increase, less cooling time is available between print lines, and the different energy requirements of cool and hot heating elements become greater. Overheating of a heating element degrades print quality and also can cause destruction of the heating element.
Thus, the printing process requires a precise control of heating element temperature to achieve optimum print quality and, therefore, some means of individually controlling printing element energy is required. The digital nature of current printing head designs makes this control difficult, since all active heating elements receive the same voltage and the same strobe signal ON time. The most common control approach involves loading and strobing the printing head multiple times for each print line. Thus, a hot heating element (one that has recently printed) may be activated for only one load and strobe cycle, while a cold heating element may be activated on every load and strobe cycle for the current print line. A digital history memory is used to store the data from past print lines. This stored data can then be used to determine how long it has been (in terms of print lines) since a heating element has been activated and for how many strobe cycles it should be activated to achieve optimum printing temperature. In general, the larger the history memory and the more load and strobe cycles per print line, the better the heat control and the better the print quality. Up to seven line history memories and four head load cycles per print line have been used. The complexity, speed and cost of such circuitry can be considerable.
In order to eliminate the need for history memories and multiple head loads, a system has been devised to provide for each heating element a thermal control circuit to effectively vary the length of the strobe signals which control the heating of each heating element. Such a system is disclosed in U.S. Pat. No. 4,330,786 which provides an integrating circuit of a resistor and capacitor to electrically simulate the change of temperature of the heating element. The capacitor voltage is compared to a reference which corresponds to a predetermined temperature, and when it exceeds the reference it activates a circuit for turning off the drive transistor. But this temperature control circuit is relatively complicated and provides a control signal which is only indirectly related to the temperature of the heating element.